Alloys of Tin | Metals | Industries | Metallurgy

The following points highlight the two main types of alloys of tin. The alloys are: 1. Solder 2. Bearing Metal.

1. Solder:

It is used for joining two metals. It melts at a lower temperature than the metals to be joined. Soft solder consists of tin and lead in various properties.

Three commonly used solders are:

 

Tin-lead solders are called soft solders. Sometimes cadmium and bismuth are partly substituted for tin to make a solder for wetting copper and brass.

Hard solders used for joining copper, brass etc., contain copper and zinc with little tin.

Other Solders:

Silver solder consists of mainly silver with a little of copper and zinc. Its use is desirable where lower melting point justifies higher cost. Gold solder consists of gold with a little copper and silver.

Solder should not have more than 0.15% copper and a total of less than 0.10% other impurities (except bismuth) with no aluminium.

2. Bearing Metal:

It is a tin-antimony-copper alloy and is used for bearings. Bearing should be made from a material which is tough and hard. Tin reduces brittleness and increases compression strength.

The composition of Babbitt metal is given below:

Tin ………70 to 90%,

Antimony ……… 7 to 24%,

Copper ………. 2 to 24%.

Babbitt metal is a general term used for soft-lead or tin-base metals which are used as cast liners in bronze or steel backing. In general, Babbitt metals are used in preference to the bronzes for higher speeds and fluctuating loads but are less proof against abuse.

These have excellent embedability and conformability characteristics. Embedability is the ability of a metal to embed in itself foreign particles, and conformability is the ability to deform plastically to compensate for irregularities in bearing assembly. The tin-base alloys compared to the lead-base alloys have better resistance to corrosion in acidic oils but have considerably higher cost.

For low-speed operation under high pressures such as bridge bearings and expansion plates, hard bronzes containing upon 20% tin are used. These can be used only with a mating surface of hardened steel and under conditions of proper alignment. For lower loads, the tin content may be decreased to 10% or less. Under conditions where alignment and lubrication are to be poor, lead is often added to bronzes.

The alloy of 80% copper, 10% tin and 10% lead is most commonly used for general machinery. Alloy of 88% copper, 10% tin and 2% zinc has good casting properties and is used for housings, etc. cast with integral bearings. Chill casting gives a finer structure and is preferable to sand casting for the best bearing properties. Alloy with 70% copper, 16% tin and rest lead is very useful for higher pressures.

In internal combustion engines for higher loads, straight copper lead alloys with 21% lead, and balance copper with small amounts of nickel or tin are used. Because of the softness of the alloy, it is applied at a high temperature in a very thin layer (0.50 mm) of a steel backing strip which is subsequently formed to fit the journal or backing.

Aluminium bronzes containing 8 to 10% aluminium with 3.6% iron are used for wearing surface for heavy duty. These are extensively used for bushings, guides, gears etc., and do not withstand conditions of poor lubrication.

Other Bearing Alloys:

Bearing alloys form a separate class of alloys.

There are several types of bearing alloys:

(i) Tin Base (Babbitt):

These are available in several compositions.

(ii) Lead Base Bearing Alloys:

These have an average 4% tin. 9% antimony and balance lead. Hardening compound in both lead base and tin base alloys is Sn. Sb. (tin-antimony) which occurs as cubes distributed throughout the soft ma­trix.

(iii) Cadmium-Nickel Bearing Metals:

It contains 1.25 to 3% cadmium and balance nickel. Compound NiCd₇ acts as hardening agent. These retain their bearing properties upto higher temperatures. These do not withstand corrosion resistance in acidic oils.

(iv) Silver Bearing Metals:

It contains 4% lead and shows the lowest coefficient of friction against iron having any combination of nickel.

(v) Copper Base Heavy Duty Bearing Metals:

These are generally mixtures of copper with 10 to 30% lead which gives antifriction qualities to the alloy. A composition of such alloy is 80% copper, 10% silicon and 10% lead.

As the main component in all types of antifriction bearing alloys is tin (Sn), these are designated with letters Sn followed by the symbol of other elements and percentages thereof in the order of importance. For example, SnSb 10 Cu 5 means an antifriction bearing alloy containing 10% Sb, 5% Cu and rest, i.e. 85% Sn.

Sometimes the percentage of Sn is specified, but percentage of lead is not given e.g., Sn 6 Sb 15Pb is an alloy containing 6% Sn, 15% Sb and 79% Pb. I.S.: 25-1966 designates the antifriction bearing alloys according to the percentages of tin (Sn) in them and that percentage is called the grade.

For example, it specifies following grades, 90, 84, 75, 69, 60, 20, 10, 6, 5, 1 whose designations are Sn 90 Sb 7 Cu 3, Sn 84 Sb 11 Cu 5, Sn 75 Sb 11 Pb, Sn 69, Zn 30, Pb, Sn 60 Sb 11 Pb, Sn 20 Sb 15 Pb, Sn 10 Sb 14 Pb, Sn 6 Sb 15 Pb, Sn 5 Sb 15 Pb, and Sn 1 Sb 15 Pb, respectively.

Grades 90 and 84 are used for lining of petrol and diesel engine bearings, cross heads in steam engines and other bearings used at high speeds. Grade 75 is used for repair jobs in mills and marine installations. Grade 69 is used for under-water applications.

Grade 60 is used for lining for bearings required for medium speed applications, such as centrifugal pumps, circular saws, electric motors etc. Grades 20 and 10 are used for low speed bearings, such as pulp crushers, concrete mixers and rope conveyors. Grade 5 is used for mill shafting’s, railway carriage and wagon bearings; and grade 1 is used as a thin line overlay on steel strips.